1. Field of the Invention
The present invention relates to a method for manufacturing an SOI (silicon on insulator) substrate wherein an SOI layer is formed on an oxide film using hydrogen ion-implantation technique.
2. Description of the Related Art
A method has been proposed for manufacturing an SOI substrate (for example, see Patent Document 1). The method comprises forming an oxide film on a surface of a first silicon substrate, implanting a high concentration of hydrogen atoms into the interior of the silicon substrate to form an ion-implantation zone at a specified depth from the surface of the substrate, bonding the first silicon substrate over a second silicon substrate to form a laminated assembly, heating the laminated assembly to 500° C. or higher, thereby separating the first substrate assembly from the second substrate assembly with the hydrogen ion-implantation zone at a boundary, and forming a semiconductor SOI layer on a surface of the second substrate. According to this method, it is possible to manufacture an SOI substrate comprising a second silicon substrate, an oxide film formed thereon and serving as a buried oxide film, and a semiconductor SOI layer formed on the top of the oxide film.
However, according to the conventional method described above for manufacturing an SOI substrate, the first silicon substrate is separated at the hydrogen ion-implantation zone, and thus the resulting SOI substrate suffers from crystal defects on the cleaved face. To solve this problem, a treatment consisting of removing a superficial layer having crystal defects from the cleaved face was suggested, and, as one of such treatments, a CMP (chemical mechanical polishing) method was proposed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-211128 (claims)
According to the aforementioned conventional method for manufacturing an SOI substrate, ions forming the hydrogen ion-implantation zone contain reactive ions that may have a damaging effect on the resulting SOI layer. Namely, since the reactive ions serve as an etchant and etch the surface of the SOI layer formed on the oxide film by the heat treatment for separating the first silicon substrate at the ion-implantation zone, they will inflict damage on the SOI layer, and particularly on its superficial layer which remains a problem to be solved. In addition, in the conventional CMP method, it is impossible to maintain the uniformity of the thickness over the entire surface of the SOI layer, because the entire layer is turned into a thin membrane at one time. Also, the amount removed from the SOI layer surface is not necessarily uniform. If the initial thickness of the SOI layer shows a variation over the entire layer, the variation may cause the degraded uniformity over the entire layer.